The present invention relates to insect electrocution products and, more particularly, to an improved insect electrocution product utilizing a switchable, dual primary coil combination high voltage transformer and flourescent tube ballast.
Conventional electric insect killers generally require the use of two devices working in concert: a flourescent tube with suitable phosphors to attract the insects, and a source of high voltage electricity to destroy them. The flourescent tube and high voltage power supply both employ a similar inductive component in their circuits. The flourescent tube or lamp requires a ballast to control the magnitude of the electric current flowing through the tube. The most common ballast is a simple inductor consisting of a laminated steel core surrounded by one or more windings of varnished copper wire. The required level of control is determined by three parameters: (a) the number of turns of wire, (b) the amount of iron or steel in the core, and (c) the dimension of a "gap" across the magnetic lines of flux.
There are many different designs of ballast circuits, but they generally fall into one of three categories depending on the means used to "flash" or ignite the tube when first turned on. The simplest of these, called the "preheat" circuit, is commonly used for the smaller flourescent tubes found on desk lamps and bathroom cabinets. In this circuit, a manually-operated switch or automatic device called a starter is used to create a momentary current through the filaments at each end of the tube. This heats the filaments, producing a "space charge" of electrons which lights the lamp when this preheat current is interrupted.
In order to achieve the proper control of the current during both the preheat phase and the operating phase, a ballast of a given physical size is required to meet the parameters outlined above. If this ballast is made smaller, (that is, if less steel in a smaller frame size is used), additional wire turns can be put on to compensate but--for any given number of turns--either the preheat current will be too high or the operating currents will be too low. Since tube wattages vary, e.g., 15 watt, 25 watt and 40 watt, it would be desirable to use a single purpose ballast for a wide range of tube wattages.
In recent years, the flourescent tube ballast and high voltage transformer have been combined into a single component. These devices use the same standard, cruciform-type lamination that has been used throughout the industry for the current-limited high voltage transformer; however, by creating an appropriate gap between the inner and outer laminations, the primary coil can also be used as the ballast for the tube. Unfortunately, this arrangement is not able to achieve the same degree of control over the several parameters of flourescent tube operation as a conventional, single-purpose ballast. Within the limit of 500 milliamperes, however, the span of control of such a device having the same lamination size and stack thickness as a conventional high voltage transformer is adequate for commercial purposes.
Beyond this limit, the span between operating and preheat current levels is too great for reliable operation. Regaining such control would require a substantial increase in lamination size or stack thickness, thereby decreasing the cost advantages over using two discrete components.
Since the operating level of commercially important 40 watt flourescent tubes is up to 800 milliamperes, it is desirable to reduce the span to that of a conventional ballast at these current levels.
It is accordingly, a general object of the present invention to provide an improved combination high voltage transformer and flourescent tube ballast for insect electrocution products.
It is a specific object of the invention to provide a switchable, dual primary coil ballast/step-up transformer that accomodates a wide range of flourescent tube wattages.
It is a feature of the invention that one of the dual primary coil provides increased impedance only during the starting operation of the flourescent tube.